Stabilization of mercury vapor discharge lamps

ABSTRACT

A method for prestabilizing mercury-vapor discharge lamps, such as fluorescent lamps. After lamp assembly, the lamp is temporarily operated while at the same time cooling, to a temperature lower than its normal intended operating temperature, a small area of the bulb, at the normal &#39;&#39;&#39;&#39;cold spot&#39;&#39;&#39;&#39; location where the mercury will condense during lamp operation. This causes the mercury to quickly condense and adhere to the phosphor and bulb at the cold spot. The lamp may then be shipped and put into use without the need for any appreciable further stabilization.

United States Patent 11 1 Beck et al.

[ June 28, 1974 STABILIZATION OF MERCURY-VAPOR DISCHARGE LAMPS [75]Inventors: Charles E. Beck, Chesterland; D.

Richard Zatyko, Richmond Heights, both of Ohio [73] Assignee: GeneralElectric Company,

Schenectady, NY.

[22] Filed: May 10, 1972 [21] Appl. No.: 252,341

[52] US. Cl....- 313/44, 313/34 [51] Int. Cl. HOlj 9/44 [58] Field ofSearch... 313/34, 44, 1.09, 174; 315/1 12 [56] References Cited UNITEDSTATES PATENTS 2,966,602 12/1960 Waymouth et a1. 313/44 X 3,205,3949/1965 Ray 313/109 3,284,664 11/1966 Morin et 211.. 313/109 X 3,331,9777/1967 Wainio 313/34 3,339,100 8/1967 Menelly 313/174 X OTHERPUBLICATIONS Underwood et 211., External Control of Mercury Pressure ofFluorescent Lamps Illuminating Engineering, Vol. LV, No. 1, January,1960, pp. 47-55 Beck et al., Light Output of 1.5 Ampere Fluorescent LampDesigns Illuminating Engineering, Vol. LX111. No. 4, April, 1968, pp.167-175 Primary Examiner-Herman Karl Saalbach Assistant ExaminerRichardA. Rosenberger Attorney, Agent, or Firm-Norman C. Fulmer; Henry P.Truesdell; Frank L. Neuhauser [5 7] ABSTRACT A method for prestabilizingmercury-vapor discharge lamps, such as fluorescent'lamps. After lampassembly, the lamp is temporarily operated while at the same timecooling, to a temperature lower than its normal intended operatingtemperature, a small area of the bulb, at the normal cold spot locationwhere the mercury will condense during lamp operation. This causes themercury to quickly condense and adhere to the phosphor and bulb at thecold spot. The lamp may then be shipped and put into use without theneed for any appreciable further stabilization.

5 Claims, 4 Drawing Figures l STABILIZATION OF MERCURY-VAPOR DISCHARGELAMPS BACKGROUND OF THE INVENTION The invention is-in the field ofmercury-vapor dis charge lamps, such as fluorescent lamps, and morespecifically relates to the stabilization of the mercury pres sure insuch lamps.

When a mercury-vapor discharge lamp is first operated, it goes through aperiod of unstabilized operation, accompanied by reduced light output,during which time mercury droplets in the lamp evaporate and form excessmercury vapor which condenses onto the coldest spot in the lamp untilequilibrium is reached between the mercury vapor pressure and thecondensed mercury at the cold spot." A typical size of the cold spot isabout one-eighth to one-fourth of an inch in diameter. A desirabletemperature of the cold spot is 40 C, causing a mercury vapor pressurein the lamp of about 6 microns, which causes maximum light to beproducedby the phosphor in the lamp. A certain amount of condensed mercury isdesirable in the lamp -to insure against depletion of mercury, and hencereduction of mercury vapor pressure, during the life of the lamp.

FIG. 1 of the drawing is derived from FIG. 2 of Light Output of 1.5Ampere Fluorescent Lamp Designs in Photometry and Use by Beck, Gabrieland Aicher, on pages 167-175 of Illuminating Engineering, Vol. LXIII,No. 4, April 1968. It shows that, during initial operation of a T12fluorescent. lamp at 1.5

amperes (solid-line curve) thelightoutput is reduced. to 85- percent ofits final value for nearly fifty hours,

until mercury stabilization occurs. The dashed line shows the lightoutput for subsequent restarts in which the condensed mercury remains atthe cold spot of the lamp and hence no substantial amount of time isrequired for further stabilization.

The location of the cold spot" of the lamp, at which the mercury vaporcondenses, depends on lamp design. In a long lamp have short lampmounts, the cold spot will be at the center of the lamp bulb. The coldspot will be at an end of the lamp bulb if the lamp mount at that end ismade long enough. Further details of lamp design and cold spot locationsare described in External Control of Mercury Pressure of FluorescentLamps and Its Application to Luminaires by Underwood and- Beck, asix-page published paper presented at the National Technical Conferenceof the Illuminating Engineering Society, Sept 7-1 l, 1959, in SanFrancisco, Calif. This paper also describes external control and coolingof the cold spot by means of cooling tins, air jets, and other devices,when a lamp is operated in a high ambient temperature or at increasedcurrent and hence increased temperature,- the purpose of such ex-.ternal cooling being to maintain the cold spot at optimum operatingtemperature during operating life of the lamp so as to insure optimummercury vapor pressure and light output.

Now referring again to FIG. 1 of the drawing, it will be evident thatthe many hours of reduced light output during initial operation of amercury-vapor lamp. caused by the mercury stabilization process justdescribed, is particularly undesirable in certain applications. Forexample, document copying machines such as xerographic machines utilizea mercury-vapor fluorescent lamp to scan and/or illuminate the documentto be copied, and the quality of the copies is dependent on thebrightness of the lamp. The reproduced copies are of inferior qualitywhen made during the initial hours of lamp operation and stabilization.Of course, the lamp may be operated during the initial stabilizingperiod in an idle machine, which is costly and timeconsuming. Operatingthe lamps at the factory or elsewhere until mercury stabilization isachieved, also is costly and time-consuming, and is not often successfulbecause subsequent shipping or handling is likely to cause the condensedmercury to leave to cold spot and roll to other places in the lamp,whereupon the low light output stabilization period will again occur inwhole or in part.

From the foregoing, the need is apparent for a method of stabilizing, orprestabilizing, mercury vapor lamps quickly and permanently.

SUMMARY OF THE INVENTION Objects of the invention are to provide animproved method of stabilizing mercury vapor lamps, and to provide amethod of prestabilizing mercury vapor lamps relatively quickly andpermanently.

The invention comprises, briefly and in a preferred embodiment,prestabilizing a mercury-vapor lamp by temporarily operating the lampwhile at the same time cooling asmall area of the lamp bulb, at thenormal cold spot location where the mercury will normally condenseduring lamp operation to a temperature lower than its normal intendedoperating temperature. This method causes the mercury to condensequickly (typically in l to 2 hours) at the coldv spot, and the methodalso advantageously causes the mercury to adhere to the phosphor and thebulb at the cold spot. The lamp may then be shipped and put into useand, due to the aforesaid prestabilization, the lamp will quicklyoperate at full normal brightness without the need for any appreciablefurther stabilization time period. The cooling of the cold spot may beachieved by an air jet, cooling fins, or other convenient means. If thelamp has a reflector coating between the bulb and the phosphor coating,as is the case with a typical reprographic lamp, preferably thereflector coating is removed or deleted at the area of the cold spot, sothat during the prestabili'zation the mercury can more readily penetratethe phosphor and come into contact with the glass bulb (or with aprecoat on the bulb if such is employed). A layer of indium, sodiumsulfate, or other mercury-wettable material at the cold spot, preferablybetween the phosphor layer and the bulb, improves adhesion andentrapment of the mercury "at the cold spot. Also, making the phosphorlayer thicker, and/or of coarser 'or larger granules at the cold spotarea, improves adhesion and entrapment of the mercury so that it will beless likely to become dislodged during subsequent handling, shipv ping,and installation in equipment.

BRIEF DESCRIPTIONOF THE DRAWING DESCRIPTION OF THE PREFERRED EMBODIMENTTo better facilitate understanding of the invention, mercurystabilization characteristics of prior art lamps I will now bedescribed, with reference to FIG. 1. In FIG.

1, the vertical axis 1 1 represents relative light output of a mercuryvapor fluorescent lamp, and the horizontal axis 12 represents operatingtime of the lamp, this time axis being separated, for clarity, into afirst-part operating time of to 20 minutes, and a subsequent continuingoperating time of from 2 to 250 hours. The solidline curve 13 representslight output with respect to initial operating time of a T12 fluorescentlamp operating at 1.5 amperes, and the dashed-line curve 14 representsthe same lamp when operatedsubsequently provided that the lamp has notbeen jostled so as to dislodge the mercury that has condensed at thecold spot of the lamp. When a lamp is initially operated, its lightoutput exceeds the normal 100 percent light output by about percent fora minute or two, as shown by curve 13 in FIG. 1 (during this period, thelamp is warming up and the mercury vapor pressure is increasing throughits optimum pressure of-6 microns, and then considerably exceeds thispressure), and then the light output of the lamp reduces to about 85percent of its final operating value and continues at this decreasedlight output value for a considerable number of hours, for example upto'50 or more hours, while excessmercury vapor in the lamp is travelingtoward and condensing at the cold spot of the lamp. When all of theexcess mercury vapor in the lamp has condensed at the coldspot, mercuryvapor equilibrium is reached and the mercury vapor pressure in the lampis a function'of the temperature of the cold spot. Preferably, the coldspot is about 40 C, which causes the mercury vapor pressure in the lampto be about 6 microns, which is optimum vapor pressure for causing the100 percent relative light output of the lamp.

If the lamp remains in position and is not jostled, the condensedmercury remains collected at the cold spot of the lamp after the lamp isturned off, and when the lamp is subsequently turned on, its brightnessoutput follows the dashed-line curve 14 and the light quickly assumesits normal I00 percent light output since no further mercurystabilization is required. If, however, the lamp is jostled or shippedfrom one location to another, or removed from a fixture or equipment andreinserted, the condensed mercury globule is likely to become dislodged,and mercury will roll around and scatter within the bulb, whereuponsubsequent operation of the lamp requires a repeat of some or all of thehours of stabilization time. I

, In the embodiment of the invention shown in FIG. 2, a mercury vaporreprographic fluorescent lamp 16 comprises an elongated soda lime silicaglass tube 17 of circular cross-section. The lamp is provided with theusual electrodes 18, 19 at each end thereof, respectively supported byinlead wires carried in stems 21, 22 of glass which are respectivelysealed to the two ends of the envelope tubing 17. Bases 23, 24 ofconventional design are attached to the ends of the glass envelope 4 17.As shown, one of the glass stems 22 is longer than the other, so thatduring operation of the lamp the end of the envelope H7 at which thelonger stem 22 is located, will be the relatively coldest part of thelamp. During manufacture, various layers of material are coated insidethe envelope tubing 17, as will be described, and then the lamp isprocessed and filled with an inert gas such as argon or a mixture ofargon and neon, at suitable pressure, and also is provided with a smallquantity of mercury, at least enough to provide a low vapor pressure ofabout 6 microns during operav tion of the lamp under normal operatingconditions.

As best shown in FIG. 3, the inner surface of the glass envelope I7 isprotectively coated with a thin, clear film 26 consisting of titaniumdioxide. In one way of forming this layer, a metallo-organic compound oftitanium such as tetrabutyl titanate or tetraisopropyl titanatedissolved in an appropriate solvent such as butyl alcohol or butylacetate, is applied to the glass. The solvent evaporates quickly and thetitanate is left deposited upon the inner surface of the glass tube.Moisture from the air hydrolizes the titanate almost as fast as thesolvent evaporates, forming titanium dioxide which remains'as a verythin clear continuous film in a thickness from 0.002 to 0.02 microns.

The lamp then is provided with a reflector coating 27, which may be aparticulate coating of titanium dioxide having a particle size less than1 micron, for instance centering on about 0.3 micron which is about halfthe median wavelength of the visible spectrum. The titanium dioxide (TiOmay be applied as a suspension in a solution of ethyl cellulose in anorganic solvent to serve as a binder, the suspension being drawn up intothe tube while supported vertically and then allowedto drain out.Thereafter the tube is lehred in order to decompose and drive out theorganic binder Alternatively, the reflector coating may consist ofmagnesium oxide (MgO). A phosphor coating 28 is applied over the insidesurface of the reflector coating 27, and may consist of calciumhalophosphate, applied as a suspension in a solution of nitrocellulosein butyl acetate which is drawn up into the tube and allowed to drainout.

A transparent aperture 29 is then formed along the side of the lamp, byscraping away the reflector and phosphor coatings over an area to formthe elongated clear window or aperture 29, as shown. The clearprotective layer 26, however, remains since it is extremely adherent tothe glass envelope 17. Instead of scraping material away to provide theaperture 29, it, may be formed by applying the reflector and phosphorcoating materials, in succession, into the tubing while supportedhorizontally and rocked back and forth about its elongated axis to coatthe bulb only over the desired area, as taught in US. Pat. No. 2,892,440Fulton et a1.

Prior to applying the phosphor layer in the lamp as has been described,a small area of the reflector 27 may be removed at the cold spot"location in the lamp,

. which, in the lamp illustrated, is near the end of the envelope atwhich the longer stem 22 is sealed, and at the side of the lamp awayfrom the aperture window 29, as indicated by the numeral 31 in FIG. 3.The size of the scraped-away reflector material at 31 may beapproximately one-eighth to one-fourth of an inch diameter, or a squareshape approximately one-eighth to one-fourth inch on the side, or anyother easily scraped-away shape of the same general size or larger.Alternatively, the same result may be achieved by not coating the coldspot region. The layer of phosphor 28, when subsequently applied, fillsin the cleared-away area at 31, as shown in FIG. 3. Alternatively to theforegoing procedure, the phosphor layer 28 is applied over the reflectorand then both the phosphor and reflective material 27 are scraped awayat the area 31, whereupon additional phosphor or other mercury-absorbingmaterial is applied in the area thus scraped away. In a modification ofthe invention, before phosphor is applied at the area 31, a layer ofmercury-wettable material 32, such as indium or sodium sulfate, isapplied to the envelope tubing 17, or to the protective coating 26, atthe area 31, and then a layer of phosphor 28 is coated over the innersurface of this mercury-wettable material.

After the lamp 16 is assembled, and filled with gas and dosed withmercury, all as described above, and

sealed off and with the bases 23, 24 either attached or not yetattached, and in accordance with the invention, the lamp isprestabilized as follows. The lamp is operated temporarily, preferablywith normal operating current, while at the same time the cold spot area31 is cooled by external means, such as by an air jet, cooling fins,.orother suitable means, to a temperature lower than its normal intendedoperating temperature. The drawing shows a cooling fin arrangement 36comprising a cooling block 37 provided with radiation fins 38, andshaped to fit the outer contour of the envelope tubing 17 at the coldspot area 31, and held in place by means of a pair of spring-wire clips41, 42 which partially encircle the outer periphery of the tubing 17 ingripping relationship thereto. The lamp is thus operated with externalcooling of the cold spot area 31 until the excess mercury vapor in thelamp has migrated toward and condensed atthe cold spot area 31. It isfound that this procedure stabilizes the lamp much more quickly thanwould occur in normal operation, and it has been further found that thisprocedure provides the very beneficial result of sufficiently adheringthe condensed mercury globule 46 at the cold spot re-' gion, so that thelamp may subsequently be handled, shipped, and installed in equipment,with considerable likelihood that the mercury globule 46 will remain inposition at the cold spot area 31, whereupon the lamp will beimmediately usable at full normal brightness, without having to undergoa time period of reduced brightness while mercury stabilization occurs.

FIG. 4 is a plot of curves, generally similar to FIG. 1, showing thequick prestabilization of the mercury achieved by the invention. In FIG.4, the vertical axis 48 represents relative light output and thehorizontal axis 49 represents lamp operating time. The scale of relativelight output on the vertical axis 48 is the same as that of verticalaxis 11 in FIG. 1; however, as will be readily seen, the time scale ofthe lamp operating time horizontal axis 49 is considerably shortenedwith respect to that of lamp operating time axis 12 of FIG. 1. In FIG.4, the solid-line curve 51 represents the light output of the lamp 16(which, in this case, is a T12 lamp operated at 1.5 amperes, the same asthat used to obtain the curves in FIG. 1) during the prestabilizationprocess. .As shown, during prestabilization the lamp operates at a lowoutput light level of about 85 percent of normal 100 percent lightoutput, for a period of only 1 to 2 hours, whereupon the excess mercuryvapor presof the lamp. The external cooling means 36 is then removed; ifleft on the lamp, the mercury vapor pressure would be less than optimumand the light output would be reduced. Thereafter the cold spot 31 is atits normal intended operating temperature and the lamp operates atnormal percent light output. In subsequent lamp operation, such as whenthe lamp has been shipped and installed in equipment, the light outputassumes that of the dashed line curve 52, i.e., the lamp assumes normal100 percent light output immediately after a short one or two minuteperiod of slightly increased light output. Thus, no time need be wasted,after the lamp is installed in equipment, for stabilizing the mercury.This achievement is due to the fact that, in accordance with a featureof the invention the prestabilization of mercury as described abovecauses the mercury to adhere to and remain at the cold spot area of thelamp, during I handling and shipping and installation, so there is nosubstantial amount of loose mercury elsewhere in the lamp that must bestabilized into the cold spot area during operation of the lamp.

It has been found that only a slight amount of external cooling of thecold spot region of the lamp, to render the cold spot slightly colderthan its normal intended operating temperature during theprestabilization process, is sufficient to achieve quick condensationandfirm adherence of the excess mercury in the lamp at the cold spotregion 31. It has further been found that external cooling'of the coldspot region 31 to a considerably lower temperature, such as by applyingdry ice externally to this'cold spot region 31, quickens theprestabilization somewhat, but at greater expenseand inconvenience, andtherefore the cooling fin technique described above is a preferredmethod of externally cooling the cold spot area 31. For a lamp designedto operate with an optimum cold spot temperature of 40 C, externalcooling to reduce the cold spot temperature to about 30 C or 35 C duringthe prestabilization, produces satisfactory results. Theprestabilization time period can be shortened somewhat by operating thelamp at increased operating current, and hence increased operatingtemperature, during the pre stabilization period.

' It is believed thatthe prestabilization in accordance with theinvention causes the mercury to be absorbed and trapped into thephosphor 28 and/or adhered to the glass surface, or protective coating,or mercurywettable coating 32, for reasons not fully understood atpresent. Perhaps the temperature gradient produced by the spot-coolingcauses the mercury to be pulled through the phosphor and/or TiO or othercoating materials, toward the glass bulb. The process should also beuseful for other metallic-vapor discharge lamps.

The invention has been particularly described in connection with amercury-vapor fluorescent reprographic lamp; however, the invention alsocan be utilized with beneficial results on other types of fluorescentand mercury-vapor lamps.

In applying the invention to prestabilization of hightemperature lampsintended to be operated with cooling fins or other cooling means at thecold spot (for maintaining the cold spot at optimum temperature such as40 C), the prestabilization is achieved by super cooling the cold'spotto reduce its temperature to a value lower than the intended operatingtemperature. This can be achieved by using larger cooling fins, or

with forced-air cooling of the fins, or dry ice, for example.

While preferred embodiments and modifications of the invention have beenshown and described, various other embodiments and modifications thereofwill be apparent to those skilled in the art, and will fall within thescope of invention as defined in the following claims.

What we claim as new and desire to secure by letters Patent of theUnited States is:

1. A method of prestabilizing. prior to use thereof, a metallic-vaporreprographic lamp of the type having a cylindrical bulb and containing acoating of reflector material within the bulb, the reflector materialbeing coated interiorly at least in part with phosphor mate-- rial, saidlamp being designed so that during normal operation thereof excessmetallic vapor material conoperating said lamp and simultaneouslytemporarily ap- 8 plying cooling means for cooling said cold spot regionto a temperature lower than the intended normal operating temperaturethereof for a time sufficient to cause a desired amount of the metallicvapor material to condense at said cold spot region, and removing saidcooling means so that in subsequent lamp operation said cold spot regionwill be at said normal operating temperature.

2. A method as claimed in claim 1, in which said time terial in the lampto condense at said cold spot region.

said lamp at the region of said cold spot.

2. A method as claimed in claim 1, in which said time ofprestabilization operation and cooling is sufficiently long to causesubstantially all excess metallic vapor material in the lamp to condenseat said cold spot region.
 3. A method as claimed in claim 1, in whichsaid cold spot region is temporarily cooled to a temperature severaldegrees Centigrade lower than the normal intended operating temperaturethereof.
 4. A method as claimed in claim 1, in which said metallic vaporis mercury.
 5. A method as claimed in claim 1, in which said coolingmeans is a cooling fin means applied against said lamp at the region ofsaid cold spot.